What is stainless steel pitting corrosion?

Strictly speaking pitting in stainless steel is not corrosion in the same way as ordinary steel rusts. Stainless steel is corrosion resistant by virtue of the chromium and nickel content in the alloy which rapidly oxidises and forms a coating of chromium oxide. This coating is transparent and impervious to oxygen and known as passivation. This effectively prevents further oxidisation. Stainless steel needs the constant presence of oxygen to remain corrosion resistant because should the passivation layer be damaged it needs oxygen to re-oxidise. Where stainless steel is encased in a material, particularly if there is high salinity present, pitting erosion will occur. Pitting erosion is essentially galvanic, that is exactly the same process that erodes a sacrificial hull anode with erosion of electrons from an anodic surface surface within an electrolyte or saltwater. When the passivation layer is damaged the surface becomes active. There is a potential difference of up to 0.4 volts between active 316 Stainless Steel and passive 316 Stainless Steel so it forms a galvanic cell and the damaged anodic surface erodes electrons into the electrolyte

Pitting erosion is, as its name implies, deep pitting rather than the uniform surface corrosion found on mild steels. Due to the tiny size of the anodic pit compared to the surrounding metal which is cathodic, galvanic action results in highly concentrated aggressive erosion of the metal. This kind of corrosion is extremely insidious, as it causes just a small loss of material with little evidence on its surface, while it erodes deep into the structure of the metal. It can also lead to stress corrosion fracturing which is the propagation of cracks in a corrosive environment. This can cause the sudden failure of metals that are subjected to a tensile stress.